The transmission mechanism of an automatic transmission for an automotive vehicle has incorporated therein a plurality of friction units which are to be selectively made operative and inoperative to make shifts between various forward and reverse drive gear ratios and portions, as is well known in the art. In an automatic transmission having three forward drive and one reverse drive gear ratios, such frictional units typically include a high-and-reverse clutch, a forward drive clutch, a brake band, and a low-and-reverse brake. Of these transmission clutches and brakes, the forward drive clutch in particular is maintained coupled throughout the conditions in which the gear ratios in the automatic and manual forward drive ranges are to be selected and established in the transmission mechanism. If all the other frictional units than the forward drive clutch are held inoperative, the transmission mechanism is conditioned to produce the first gear ratio in the automatic forward drive range in cooperation with a one-way clutch also incorporated into the transmission mechanism. If the brake band is put into operation additionally to the forward drive clutch, then the second gear ratio in the automatic or manual forward drive range is selected depending upon the position to which the manually operated transmission gear shift lever has been moved. If the brake band is thereafter released and, in turn, the high-and-reverse clutch is actuated to couple, a shift is made from the second gear ratio to the third gear ratio in the automatic forward drive range. If, on the other hand, the high-and-reverse clutch and the low-and-reverse brake are in operation concurrently with the forward drive clutch held inoperative, then the reverse drive gear ratio is selected in the transmission mechanism.
The clutches and brakes which effect the shifting between the various gear ratio conditions of the transmission mechanism are hydraulically operated by means of a hydraulic control system which is basically operated by manipulating the above-mentioned transmission gear shift lever which is usually installed on the floor, the steering column or otherwise of an automotive vehicle. More specifically, each of the clutches and brakes provided in the transmission mechanism is operated by a control fluid pressure, or line pressure, produced by a pressure regulator valve incorporated into the control system. In the hydraulic control system of a known automatic power transmission, the line pressure thus produced by the pressure regulator is developed on the basis of another control pressure, or throttle pressure, which represents the opening degree of the throttle valve provided in the carburetor of an internal combustion engine with which the power transmission is to operate in an automotive vehicle. The clutches and brakes for controlling the shifts between gear ratios are, for this reason, operative with forces which vary with the opening degree of the carburetor throttle valve.
The line pressure supplied from the pressure regulator is distributed to the transmission clutches and brakes selectively by and through a suitable number of gear shift valves which are responsive to both the throttle pressure and a third control fluid pressure, or governor pressure, which represents the road speed of the vehicle in operation. The transitive points, or shift points, at which shifts are to be automatically made between the gear ratios available are thus determined by the relationship between the vehicle speed and the opening degree of the carburetor throttle valve.
For good driveability under low load condition of the engine with small throttle opening degrees, it is desirable that an upshift from the lower gear ratio to the higher gear ratio takes place at a vehicle speed falling in a narrow range or at a predetermined vehicle speed.
As mentioned in the preceding, according to the conventional practice a line pressure is produced based on a throttle pressure so that the line pressure varies approximately with the characteristic curve of the engine torque. If a hydraulic control system including a forward clutch servo and a high-and-reverse clutch servo is operated with this line pressure, there will not take place a noticeable shock nor slippage in the case of the forward clutch since this clutch is kept energized or engaged during forward running, but there will occur a noticeable shock upon energization of the high-and-reverse clutch. The reason for the occurrence of the noticeable shift shock in the case of the high-and-reverse clutch is that the magnitude vs. throttle opening degree characteristics of this line pressure does not agree with that of a desired servo actuating pressure which is considered to be suitable for actuating the high-and-reverse clutch in shockless manner over the whole throttle opening degrees. Explaining it more, if, in design, a magnitude of the line pressure is tailored to a suitable magnitude for actuation of the high-and-reverse clutch upon its engagement during upshifting under high speed high load engine operating condition, a magnitude of the line pressure for the clutch actuation becomes excessively large under low speed low load engine operating condition, thus causing a noticeable magnitude of shift shock under this low speed low load engine operating condition. If a magnitude of the line pressure is tailored to a suitable magnitude for the clutch operation of the high-and-reverse clutch under low speed low load engine operating condition, a magnitude of the line pressure for the clutch actuation becomes excessively small under high speed high load engine operating condition, thus causing a slippage of the high-and-reverse clutch under this high speed high load engine operating condition.
According to another conventional practice, a line pressure is produced based on a throttle pressure so that the line pressure varies in proportion to throttle opening degree without precisely approximating the characteristic curve of the engine torque. If this line pressure is used, a noticeable magnitude of upshift shock under low speed low load enging operating condition is unavoidable since the line pressure is set to have a magnitude large enough, not zero, even at zero throttle opening degree for the purpose of producing a sufficiently high pressure for the lubrication and for the production of a governor pressure.
Considering a shift shock taking place upon engagement of the high-and-reverse clutch during upshifting from the second gear ratio to the third gear ratio, the magnitude of the shift shock is considered to be variable with kinetic energy differential between the clutch elements immediately before the clutch engagement. The kinetic energy differential is in part function of the engine torque and vehicle speed.
Applicants have noted that the shift shock, during upshifting, will be reduced by actuating a frictional unit to be actuated for this upshifting with a servo actuating pressure which will have a magnitude, during the upshifting at least, indicative of not only the engine torque but also the vehicle speed at the time of the upshifting.